U.S. patent number 5,466,498 [Application Number 08/140,342] was granted by the patent office on 1995-11-14 for pasteurizable, cook-in multilayer shrink film.
This patent grant is currently assigned to W. R. Grace & Co.-Conn.. Invention is credited to Roberto Forloni, Mario Paleari.
United States Patent |
5,466,498 |
Forloni , et al. |
November 14, 1995 |
Pasteurizable, cook-in multilayer shrink film
Abstract
A pasteurizable and/or cook-in multilayer shrink film, which is
characterized in that the innermost sealing layer comprises an
ethylene-butyl acrylate-maleic anhydride copolymer.
Inventors: |
Forloni; Roberto (Nerviano,
IT), Paleari; Mario (Pogliano, IT) |
Assignee: |
W. R. Grace & Co.-Conn.
(Duncan, SC)
|
Family
ID: |
26132580 |
Appl.
No.: |
08/140,342 |
Filed: |
October 21, 1993 |
Current U.S.
Class: |
428/36.7;
428/516; 428/910; 428/517; 428/35.7 |
Current CPC
Class: |
B32B
27/28 (20130101); B32B 27/32 (20130101); B32B
27/16 (20130101); B32B 27/306 (20130101); B32B
27/08 (20130101); B32B 7/12 (20130101); B32B
27/304 (20130101); C04B 26/04 (20130101); C04B
2111/00482 (20130101); C04B 2111/00965 (20130101); Y10T
428/31913 (20150401); Y10T 428/31917 (20150401); Y10T
428/1383 (20150115); Y10T 428/1352 (20150115); Y10S
428/91 (20130101); B32B 2305/72 (20130101); B32B
2310/14 (20130101); B32B 2310/0887 (20130101); B32B
2307/736 (20130101); B32B 2307/514 (20130101); B32B
2307/718 (20130101) |
Current International
Class: |
C04B
28/02 (20060101); B32B 27/28 (20060101); C04B
28/00 (20060101); B29D 022/00 () |
Field of
Search: |
;428/910,516,517,35.7,36.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Buffalow; Edith
Attorney, Agent or Firm: Lee, Jr.; William D. Quatt; Mark B.
Hurley, Jr.; Rupert B.
Claims
We claim:
1. A pasteurizable or cook-in multilayer shrink film, comprising an
outer sealing layer comprising ethylene-butyl acrylate-maleic
anhydride copolymer, and an outer or abuse shrink layer.
2. The film according to claim 1, wherein the film further
comprises at least one barrier layer.
3. The film according to claim 1 or 2, wherein the ethylene-butyl
acrylate-maleic anhydride copolymer comprises 75 to 98% by weight
of ethylene, 1 to 20 % by weight of butyl acrylate and 1 to 5 % by
weight of maleic anhydride.
4. The film according to claim 3, wherein the ethylene-butyl
acrylate-maleic anhydride copolymer comprises 86 to 96% by weight
of ethylene, 2 to 10% by weight of butyl acrylate and 2 to 4% by
weight of maleic anhydride.
5. The film according to claim 1, wherein the film has been
irradiated with high energy electrons.
6. The film according to claim 1, wherein the food-contacting inner
surface of said innermost sealing layer is corona treated.
7. The film according to claim 1, further comprising a first
adhesive layer, a barrier layer, a second adhesive layer and an
inner shrink layer, wherein all said layers are melt-bonded to
adjacent layers of said film.
8. The film according to claim 7, wherein said shrink layers
comprise a cross-linked ethylene copolymer, and wherein the
thickness of said shrink layers is sufficient such that the shrink
temperature of the entire multilayer film, when oriented, is
substantially controlled by the shrink temperature of said shrink
layers.
9. The film according to claim 8, wherein said shrink layers
comprise an ethylene-vinyl acetate copolymer having a vinyl acetate
content of up to about 12% by weight, or low density
polyethylene.
10. The film according to claim 9, wherein said ethylene-vinyl
acetate copolymer comprises 88 to 95% by weight of ethylene and 5
to 12% by weight of vinyl acetate.
11. The film according to claim 7, wherein said adhesive layers
comprise a chemically modified cross-linked polyethylene and have
functional groups with a relatively strong affinity for the
adjacent layers.
12. The film according to claim 11, wherein said adhesive layers
comprise an ethylene polymer modified with vinyl acetate and
anhydride functionalities.
13. The film according to claim 11, wherein said adhesive layers
comprise a graft copolymer of ethylene with vinyl acetate or an
alpha-olefin and at least one unsaturated carboxylic acid
anhydride, or a maleic anhydride modified low density
polyethylene.
14. The film according to, claim 7, wherein said barrier layer
comprises an ethylene-vinyl alcohol copolymer or a vinylidene
chloride copolymer.
15. The film according to claim 1, wherein all of said layers of
said film are irradiatively cross-linked to an extent corresponding
to an irradiation dosage sufficient to increase the resistance of
the film layers to delamination under cook-in or pasteurizing
conditions but which permits the innermost sealing layer to
function satisfactorily as a sealing layer.
16. The film according to claim 15, wherein said film is
irradiatively cross-linked to an extent corresponding to a dosage
of about 3-12 MR.
17. The film according to claim 16, wherein said film is
irradiatively cross-linked to an extent corresponding to a dosage
of about 6-8 MR.
18. The film according to claim 16, wherein said film is
oriented.
19. The film according to claim 18, characterized in that said film
is biaxially oriented to an extent corresponding to a biaxial free
shrinkage at 85.degree. C. of about 3 to 65%.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to heat shrinkable, relatively gas
impermeable, thermoplastic packaging film which can be heat sealed
to itself to form a flexible package. The invention relates more
particularly to a pasteurizable and/or cook-in multilayer shrink
film preferably comprising an outer layer, at least one barrier
layer, at least one shrink layer and an innermost sealing layer
which can be used for the manufacture of bags for food products in
which the packaged product is thermally treated for a certain
period for pasteurizing or cooking, the multilayer structure ,being
shrinkable and resistant to such conditions.
There is a need in the food packaging industry for a packaging film
from which bags can be made which are of improved structural
soundness such that they may be fully characterized as
pasteurizable and/or cook-in. Further, it is desirable to have a
precooked food product which is attractively packaged inside the
film within which it was precooked.
The term "pasteurizable" as used herein is intended to refer to
packaging material structurally capable of withstanding exposure to
pasteurizing conditions while containing a food product. It is
common practice in the food industry to apply a pasteurization
process to certain food products after packaging, In order to
reduce the bacterial load of such products, thereby improving
product freshness and extending the shelf life. Specific
pasteurization requirements tend to vary by country: however,
limiting conditions probably are submersion of the hermetically
sealed product in water at 98.degree. C. for 1 hour. Thus, for a
bag to be characterized as pasteurizable, structural integrity of
the bag must be maintained during pasteurization, i.e. the bag must
have superior high temperature seal strength and must be
delamination resistant under such time-temperature conditions.
Additionally, the packaging material should be heat shrinkable
under pasteurizing conditions so as to provide an attractively
packaged pasteurized food product.
The term "cook-in" as used herein is intended to refer to packaging
material structurally capable of withstanding exposure to cook-in
time-temperature conditions while containing a food product.
Cook-in packaged foods are essentially pre-packaged, pre-cooked
foods that go directly to the consumer in that configuration which
may be consumed with or without re-heating. Cook-in
time-temperature conditions typically refer to a long slow cooking
process, for example submersion in water at up to 90.degree. C. for
several hours. Such cook-in time-temperature requirements are
representative of institutional cooking requirements. Submersion at
80.degree. C. for 12 hours or 90.degree. C. for 8 hours probably
represent limiting cases. Under such conditions, a packaging
material properly characterized as cook-in will maintain seal
integrity and will be delamination resistant. Additionally, the
packaging film should be heat shrinkable under these conditions so
as to form a tightly fitting package and preferably should have
some tendency for product adhesion to prevent "cook-out" or
collection of juices between the surface of the food product and
the interior surface of the packaging material.
Generalizing, there are a number of requirements for a
pasteurizable, cook-in packaging material. It is the purpose of the
present invention to provide a pasteurizable, cook-in packaging
film meeting all of these requirements. First, bags made from such
film must have seal integrity under such conditions, i.e. the heat
sealed seams should resist being pulled apart during heat
shrinking. As a corollary, the film should be heat sealable to
itself. Second, such bags must be delamination resistant, i.e. the
multilayers making up the film must not separate or blister. Third,
the food contact layer of such film must qualify under the
appropriate food laws and regulations for safe food contact.
Fourth, the film must provide an oxygen and vapor barrier, i.e.
must possess a low permeability to maintain the freshness of the
food contained therein. Fifth, the film must be heat shrinkable in
hot water under these time-temperature conditions, i.e. the film
must possess sufficient shrink energy such that upon the packaged
food product being submerged in hot water the packaging film will
shrink snugly around the product contained therein,
representatively about 30-50% biaxial shrinkage at about 90.degree.
C. Sixth, the film should possess optical clarity, i.e. the film
should not become cloudy upon exposure to these time-temperature
conditions so as to maintain eye appeal of the packaged product.
Seventh, if used for cooking the film should have food product
adherence to restrict "cook-out" or collection of juices between
the surface of the contained food product and the food contact
surface of the packaging material during cook-in, thereby
increasing product yield.
In general, such a multilayer film structure will have the minimal
structure (sealing and food contact layer)/(shrink layer)/(barrier
layer)/(abuse layer), a composite structure being required to
achieve the desired composite properties of the packaging film.
A heat shrinkable, thermoplastic, barrier packaging film for making
bags which has enjoyed considerable commercial success is described
in U.S. Pat. No. 3,741,253 (issued on Jun. 26, 1973 to Brax et
al.), which relates to a multilayer film comprising a sealing layer
of an irradiated ethylene-vinyl acetate copolymer, a core layer of
vinylidene chloride copolymer, and a second outside layer of an
ethylene-vinyl acetate copolymer. In manufacturing this type of
heat shrinkable film, a tubular orientation process is utilized
wherein a primary tube of the film is biaxially oriented by
stretching with internal pressure in the transverse direction and
with the use of pinch rolls at different speeds in the machine
direction. Then the bubble is collapsed, and the film is wound up
as flattened, seamless, tubular film to be used later to make bags,
e.g. either end-seal bags typically made by transversely heat
sealing across the width of flattened tubing followed by severing
the tubing so that the transverse seal forms the bottom of a bag,
or side-seal bags in which, the transverse seals form the bag sides
and one edge of the tubing forms the bag bottom.
This type of bag is typically used by placing the food product in
the bag, evacuating the bag, gathering and applying a metal clip
around the gathered mouth of the bag to form a hermetic seal, and
then immersing the bag in a hot water bath at approximately the
same temperature at which the film was stretch-oriented, typically
about 71.degree. to 96.degree. C., hot water immersion being one of
the quickest and most economical means of transferring sufficient
heat to the film to shrink it uniformly. Alternatively, the bag may
serve as a liner of a cooking mold. One problem which has been
encountered is the failure of the bag seals at the bottom of the
bags, when the bag is subjected to a temperature of 70.degree. C.
or higher for substantial periods of time, such as experienced
during a cooking or pasteurization process.
U.S. Pat. No. 4,352,702 (issued Oct. 5. 1982 to Bornstein) covers a
pasteurizable shrink bag from tubular film having a layer of
hydrolyzed ethylene-vinyl acetate copolymer and an interior surface
layer of a polyolefin which is cross-linkable by ionizing
radiation, in with the layers being directly melt-joined without an
adhesive disposed therebetween and the film being irradiatively
cross-linked and oriented. A second irradiation treatment is
carried out on receptacles made from the film to condition their
heat seals for pasteurizing conditions.
U.S. Pat. No. 4,064,296 (issued Dec. 20, 1977 to Bornstein et al.)
is directed to a coextruded tubular film having a layer of a
hydrolyzed ethylene-vinyl acetate copolymer layer between two other
polymeric layers at least one of which being irradiatively
cross-linkable, the film being irradiated and oriented.
U.S. Pat. No. 3,595,740 (issued Jul. 27, 1971 to Gerow) discloses
oxygen barrier films having an interior barrier layer of a melt
extrudable hydrolyzed ethylene-vinyl acetate copolymer and a heat
sealing layer of an ethylene polymer or copolymer.
U.S. Pat. No. 4,469,742 (issued Sep. 4, 1984 to Oberle et al.)
discloses a pasteurizable, cook-in shrink film comprising a first
or sealing layer, preferably made of an ionomer characterized as a
metal salt neutralized copolymer of ethylene and acrylic acid or
methacrylic acid; a second or shrink layer, a third or adhesive
layer, a fourth or barrier layer, a fifth or adhesive layer and a
sixth or abuse layer, wherein all of said layers are radiatively
cross-linked to an extent corresponding to an irradiation dosage
sufficient to increase the resistance of the film layers to
delamination under cook-in or pasteurizing conditions but which
permits the first layer to function as a sealing layer. The sealing
layer being preferably made of an ionomer characterized as a metal
salt neutralized copolymer of ethylene and acrylic acid or
methacrylic acid.
U.S. Pat. No. 4,888,223 (issued Dec. 19, 1989 to Sugimoto et al.)
discloses a food-packaging material comprising a heat-shrinkable,
gas-barrier, multilayer plastic film laminate in the form of a
seamless tube, wherein the innermost, food-contacting surface has
an increased wet tension strength obtained by subjecting said
surface to a Corona discharge.
JP-A-02184437-A describes a multi-ply film for packaging of raw and
processed meat comprising an innermost layer which is made of a
polyolefin resin containing as an essential ingredient an
ethylene-ethyl acrylate-maleic anhydride copolymer.
U.S. Pat. No. 4,411,919 (issued Oct. 25, 1983, to Thompson)
discloses a meat adhering cook-in packaging comprising a flexible
plastic container being substantially conformable to a selected
meat product and having an inner meat product contacting surface of
polymeric olefin having been subjected to an energetic radiation
surface treatment in the presence of oxygen sufficient to cause
said inner surface to adhere to the meat product during cook-in,
said container having been formed from hot blown tubular film made
of polyethylene, polypropylene or ethylene vinyl acetate
copolymer.
GB-B-2 009 033 (issued Aug. 25, 1982, to Matsuoka et al.) describes
synthetic resin films for meat packaging which is made either of an
olefin resin or a vinylidene chloride resin containing
substantially no functional groups, the inner surface of which is
activated by means of a corona or glow discharge treatment to
increase meat-adhesiveness capable of withstanding qualitative
changes in the meaty surface.
U.S. Pat. No. 4,855,183 (issued Aug. 8, 1989, to Oberle) as well
discloses a multiple-layer cook-in film comprising a food contact
surface made of a polyamide and having been subjected to an
energetic radiation surface treatment provided by a high energy
electron treatment to an extent corresponding to a dosage of up to
about 12 MR.
U.S. Pat. No. 4,606,922 (issued 1986 to Schirmer) relates to a
method for enhancing yield of a cook-in package meat product that
includes first providing an adhering cook-in container including a
flexible thermoplastic envelope being substantially conformable to
a contained meat product and having an inner meat-contacting
surface of a selectively irradiated ionomer or a metal salt
neutralized copolymer of ethylene and acrylic acid or methacrylic
acid, then conforming the container about a selected meat product
and cooking the package product, whereupon the inner surface of the
envelope bonds to the meat product substantially to prevent
cook-out of fluids. Representatively, the ionomer of the inner
binding surface is Surlyn, and a typical casing or an envelope is
of the structure nylon- 6/adhesive/Surlyn.
For pasteurizable and/or cook-in multilayer shrink films for use as
packaging bags for food products, it is important that the
innermost layer being in contact with the food product contacts the
food product closely and adheres to it so that the purge or liquids
exuding from the food product do not get between the meat surface
and the bag wall to create an unsightly package appearance.
Specifically for meat it is necessary to keep the juices and
liquids within the meat so that the meat does not become dry and
its weight is not diminished. On the other hand the surface of this
layer being the sealing layer must provide a strong seal closing
the bag into which the food product has been introduced.
The problem to be solved by the present invention therefore is the
provision of a pasteurizable and/or cook-in multilayer shrink film
specifically for the manufacture of bags for food products which
provides close and superior adherence to the food product and at
the same time an increased seal strength when heat sealing the
inner surfaces of said film.
It has been found that this problem can be solved by using as the
material for the innermost sealing layer of the multilayer shrink
film an ethylene-butyl acrylatemaleic anhydride copolymer, which
according to a preferred embodiment is subjected to a Corona
treatment.
SUMMARY OF THE INVENTION
The present invention is directed to a pasteurizable and/or cook-in
multilayer shrink film from which packaging bags can be made which
maintain seal integrity, are delamination resistant and are heat
shrinkable during pasteurizing and/or cooking of a contained food
product at a sustained elevated temperature in water or steam, and
exhibit a suitable degree of adhesion to the surface of the meat
product.
Accordingly, there is provided a pasteurizable and/or cook-in
multilayer shrink film, which is characterized in that the
innermost sealing layer comprises an ethylene-butyl acrylate-maleic
anhydride copolymer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to a preferred embodiment of the present invention the
multi-layer shrink film comprises at least one shrink layer and
said sealing layer, and more preferably furthermore an outer layer
and at least one barrier layer. A most preferred embodiment thereof
comprises an outer abuse shrink layer, an outer adhesive layer, a
barrier layer, an inner adhesive layer, an inner shrink layer and
said innermost sealing layers, all layers being melt-bonded to the
corresponding adjacent layers.
A representative embodiment of the invention is a composite tubular
film having the multilayer structure (inside) A/B/C/D/C/E
(outside), wherein A is the innermost sealing layer comprising the
ethylene-butyl acrylate-maleic anhydride copolymer. B is primarily
a shrink layer. C primarily are adhesive layers, D is primarily a
barrier layer and E is an outer abuse layer. A tubular film
configuration is especially suited for bag making.
The seal material of layer A comprises the ethylene-butyl
acrylate-maleic anhydride copolymer as defined above, which is
relatively resistant to degradation in the presence of grease so
that the seal integrity of bags made from the film is maintained
during cook-in conditions and has a softening point greater than
that of shrink layer B, so that during the heat shrinkage of a bag
made from the film about a contained food product the bag seals are
not softened. According to a preferred embodiment of the present
invention the ethylene-butyl acrylate-maleic anhydride copolymer
has a softening point greater than that of the following shrink
layer and more preferably comprises 75 to 98% by weight of
ethylene, 1 to 20% by weight of butyl acrylate and 1 to 5% by
weight of maleic anhydride. According to a more preferred
embodiment said ethylene-butyl acrylate-maleic anhydride copolymer
comprises 86 to 96% by weight of ethylene, 2 to 10% by weight of
butyl acrylate and 2 to 4% by weight of maleic anhydride.
The second layer B being a shrink layer, is melt bonded to the
innermost sealing layer A and is composed of an ethylene
homopolymer or copolymer, representatively an ethylene-vinyl
acetate copolymer having a vinyl acetate content of about 12% or
less by weight, preferably 5 to 12% by weight of vinyl acetate, or
comprises a linear low density polyethylene. The term "shrink
layer" is intended to refer to the shrink controlling layer that
initiates compatible shrinkage of the overall multilayer structure.
The relative thickness of the shrink layer is selected as being
sufficient relative to that of the overall film thickness such that
the shrink temperature of the shrink layer controls the shrinkage
of the entire multilayer film, when oriented.
Barrier layer D is preferably composed of a hydrolyzed
ethylene-vinyl acetate copolymer and more preferably an
ethylene-vinyl alcohol copolymer preferably comprising 44% by
weight of ethylene and 56% by weight of vinyl alcohol and,
importantly, is not degraded during irradiative crosslinking of the
tubular film.
Adhesive layers C are melt bonded adjacent to the barrier layer to
provide delamination resistance of the barrier layer in the shrink
film under pasteurizing or cook-in conditions. The adhesive is
composed of a chemically modified polyethylene being irradiatively
cross-linkable and having functional groups with a relatively
strong affinity for the barrier material. More preferably, the
adhesive layers comprise an ethylene polymer modified with vinyl
acetate and anhydride functionalities.
The outer or abuse shrink layer E isolates the barrier layer from
adverse moisture contact and is representatively an ethylene
copolymer, preferably an ethylene-vinyl acetate copolymer having a
vinyl acetate content of 12% or less by weight or is linear low
density polyethylene. More preferably this layer comprises an
ethylene-vinyl acetate copolymer having a vinyl acetate content of
about 5 to 12% by weight, more preferably about 6.5% by weight.
All layers within the film are melt bonded to their respective
adjacent layers by virtue of full coextrusion after which the
entire multilayer film preferably is subjected to ionizing
radiation treatment to crosslink the layers. Representatively, the
film will have an overall thickness prior to orientation of about
360 to 600 .mu.m, the innermost sealing layer A will have a
thickness of about 80 to 130 .mu.m, shrink layer B about 80 to 130
.mu.m, adhesive layers C about 20 to 40 .mu.m each, barrier layer D
about 20 to 40 .mu.m, and outer or abuse shrink layer E about 140
to 220 .mu.m. After orientation the film will have an overall
thickness of about 50 to 68 .mu.m, the innermost sealing layer A
will have a thickness of about 14 to 18 .mu.m, shrink layer B about
11 to 15 .mu.m, adhesive layers C about 3 to 5 .mu. m each, barrier
layer D about 3 to 5 .mu.m, and outer or abuse shrink layer E about
19 to 25 .mu.m.
According to a further preferred embodiment of the invention the
food-contacting inner surface of said innermost sealing layer is
Corona treated, because this treatment further improves the
adhesion of the film to the food product to be packaged and the
seal-strength of the sealed bag made from said shrink film.
The tubular film of the invention can be made by a process similar
to that described in U.S. Pat. No. 4,469,742 (Oberle), cited above,
further provided that the tubular film is fully coextruded, i.e.
all layers are simultaneously coextruded, using the conventional
blown bubble technique. After cooling, the coextruded tube is
flattened and then guided through an ionizing radiation field, for
example through the beam of an electron accelerator to receive a
radiation dosage in the range of about 3-12 megarads (MR).
Irradiation by electrons to cross-link the molecules of polymeric
material is conventional in the art. Radiation dosages are referred
to herein in terms of the radiation unit "rad", with one million
rads or a megarad being designated as "MR". The degree of molecular
cross-linking is expressed in terms of the radiation dosage that
induces the cross-linking. In general, irradiation should be
sufficient to cross-link the irradiatively cross-linkable layers of
the film to increase strength of the shrink layer without
substantially diminishing elongation properties, and to provide
delamination resistance of the film during pasteurizing or cook-in
conditions. The tubular film is then cooled and collapsed after
which it is fed into a hot water tank having water at about
88.degree.-96.degree. C. to soften the film for orientation; then
it passes through pinch rolls and is inflated into a bubble and
stretched to a point where the film thickness is representatively
60 .mu.m. Suitable thickness will range from about 25 to 120 .mu.m
with a stretch ratio of about 8-15:1, which will impart a shrink
capacity of about 30-55% biaxial free shrinkage at 85.degree. C.
(by ASTM D2732). As the bubble emerges from the hot water tank it
cools rapidly in the air and then is collapsed and rolled up into
flattened tubing. It is from this tubing of this final oriented
thickness that bags are made as discussed above.
Since the barrier layer of the hydrolyzed ethylene-vinyl acetate
copolymer (EVOH) is not degraded during radiation treatment of the
entire multilayer film, the film maybe fully or simultaneously
coextruded. Full coextrusion is advantageous in that all layers of
the multilayer film are directly melt joined for enhanced
interlayer strength under pasteurizing or cook-in conditions.
In use, bags are made from the film of the invention in
conventional manner, as discussed above, to form either end-seal or
side-seal bags. Eventually, the bags are loaded with a food
product, vacuumized and sealed, and subjected to pasteurizing or
cook-in treatment in near boiling water. During this food
treatment, bags maintain good seal integrity, do not delaminate,
and heat shrink to form a neatly packaged pretreated food
product.
The sealing layer used according to the present invention is
composed of an ethylene-butyl acrylate-maleic anhydride random
copolymer. The term "random" is used in the conventional sense to
refer to a copolymer consisting of segments of at least two
monomeric units of random lengths, including single molecules,
arranged in random order. Radiation treatment causes cross-linking
of the copolymer, without significally effecting its sealing
range.
Such ethylene-butyl acrylate-maleic anhydride copolymers are
commercially available. A preferred embodiment comprises copolymers
based upon 91% by weight of ethylene, 5.5% by weight of butyl
acrylate and 3.5% by weight of maleic anhydride having a melt index
of 5 g/10 min, a melting point of 107.degree. C., a Vicat
temperature of 85.degree. C., a tensile strength at break of 12
MPa, an elongation at break of 600% and a flexural modulus of 120
MPa.
The inner shrink layer is an ethylene homopolymer or copolymer such
as linear low density polyethylene, ethylene-vinyl acetate
copolymer, or ethylene-methylacrylate copolymer. Preferably, the
shrink layer is composed of an ethylene-vinyl acetate copolymer
(EVA) having a vinyl acetate content in a range of about 5-12%,
with the orientation temperature generally decreasing and shrink
capacity increasing as the vinyl acetate content is increased.
However, the melt temperature of EVA tends to decrease as the vinyl
acetate content increases so that a content of about 12% is
limiting with a melting temperature of about 95.degree. C. for
pasteurizing applications. Irradiative cross-linking corresponding
to a dosage of about 3-12 MR provides sufficient cross-linking in
the shrink layer to enable production of the tubular film and
orienting by the blown bubble technique at economic production
rates.
The barrier layer is composed of hydrolyzed ethylene-vinyl acetate
copolymer (EVOH), preferably hydrolyzed to at least about 50%, most
preferably to greater than about 99%. The mole percent of vinyl
acetate prior to hydrolysis should be at least about 29% since for
lesser amounts the effectiveness of the hydrolyzed copolymer as a
gas barrier is substantially diminished. It is further preferred
that the EVOH copolymer have a melt flow being generally compatible
with that of the other components of the multilayer film,
preferably in the range of 3-20, more preferably in the range of
about 3-10 (melt flow being determined generally in accordance with
ASTM D 1238). The gas of main concern is oxygen and transmission is
considered to be sufficiently low, i.e. the material is relatively
gas impermeable, when the transmission rate is below 70 cm.sup.3
/m.sup.2 /mil thickness/24 hrs./atms, as measured according to the
procedures of ASTM Method D-3985. The multilayer film of the
present invention has a transmission rate below this value. EVOH is
advantageously utilized in the film of the invention since
irradiative treatment of the fully coextruded film does not degrade
the barrier layer, as would be the case for a vinylidene
chloride-vinyl chloride copolymer (saran) barrier. Most preferably,
the barrier layer comprises an ethylene-vinyl alcohol copolymer
containing 44% by weight of ethylene and 56% by weight of vinyl
alcohol.
The adhesive interlayers melt bonded adjacent the barrier layer are
composed generally of a chemically modified polyethylene being
irradiatively cross-linkable and being chemically modified by the
provision of functional groups having a strong affinity for the
EVOH copolymer of the barrier layer and which will form a strong
bond under the heat and pressure of coextrusion. Preferably, the
adhesive is a maleic anhydride grafted ethylene vinyl acetate
copolymer ("BYNEL" from DuPont de Nemours) or a maleic anhydride
grafted ethylene-alpha-olefin copolymer. Another suitable adhesive
is a low density polyethylene modified with maleic anhydride to
provide the necessary adhesion.
The outer or abuse shrink layer is provided to isolate the EVOH
barrier layer from moisture contact and thereby to prevent
degradation in barrier properties. The abuse layer is composed
preferably of an ethylene homopolymer or copolymer, generally
similar to the material of the foregoing shrink layer. More
preferably, the abuse layer is composed of ethylene-vinyl acetate
copolymer having a vinyl acetate content of about 5 to 12% by
weight, most preferably about 6.5% by weight. Alternatively, the
outer abuse layer may be the same as the sealing layer, this
configuration being appropriate for form/fill/seal packaging
wherein heat sealing is done on overlapped edge portions of a sheet
of film.
The films according to the present invention exhibit excellent
resistance to delamination and while the present invention is not
to be limited to any particular theory concerning this superior
resistance to delamination, it is believed that by irradiating the
fully coextruded structure some measure of cross-linking across the
interfaces of the various layers occurs. When the layers have been
melt joined in the coextrusion process, there is a measure of
intermingling of the melts at the layer interfaces. Thus, it is
theorized that molecules from one layer are cross-linked with those
in an adjacent layer to some extent during irradiative treatment.
Additionally, it is believed that cross-linking accounts for an
increase in viscosity, the beneficial effect being realized upon
the multilayer components being heated into their respective
softening ranges.
Furthermore, because of the use of an ethylene-butyl acrylate
maleic anhydride copolymer for the manufacture of the innermost
sealing layer an increase of the adhesion to the food products be
packaged and specifically a high protein adhesion is obtained.
Additionally, the preferred irradiation and corona treatment of the
inner surface of the innermost sealing layer provides for a
dramatic improvement of the thermal resistance of the treated
multilayer structure and at the same time provides for an increased
sealing strength. This is considered to be surprising, because
normally a Corona treatment is expected to provide a negative
effect on sealability and seal strength both at cold and hot
conditions. Therefore, the subject matter of the present invention
provides a product having unexpectedly improved properties
specifically with respect to the thermal resistance, the adhesion
of the film to the food to be packaged and the sealing strength
obtained when sealing the films via the innermost sealing
layer.
The resins or basic polymeric materials fed into the extruders to
make the tubular film of the present invention are widely available
and can be purchased from any of a number of suppliers, for example
those identified in trade publications such as Modern Plastics
Encyclopedia.
The Corona treatment referred to above is applied on the internal
surface of the tubing by means of two couples of electrodes, one
reversed versus the other in order to assure the same treatment
level on the two sides of the tubing. The electrodes are connected
to a high tension/high frequency generator preferably being
operated at for example 15 kV and 20 kHz. The Corona treatment
level of the internal surface of the innermost sealing layer should
be at least 35 dynes/cm. Such level of Corona treatment can be
achieved with the above equipment when moving the tubing with a
linear speed from 50 to 100 m/min, this range being a function of
the bag size to be manufactured.
The invention is described more in detail on the basis of the
following examples and comparative example.
EXAMPLES AND COMPARATIVE EXAMPLE
In accordance with the disclosure of the present invention
pasteurizable and/or cook-in multilayer shrink films have been
manufactured by coextruding the six layers made of the materials
referred to in the following Table 1.
TABLE 1
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outer or inner inner abuse innermost shrink adhesive barrier outer
adhe- shrink Layers sealing layer layer layer layer sive layer
layer
__________________________________________________________________________
Comparative Ethylene- EVA modified EVOH modified EVA Example
methacrylic EVA EVA acid copolym- er partly neu- tralized with Zn
Example 1 Ethylene-butyl EVA modified EVOH modified EVA acrylate-
EVA EVA maleic anhydride co- polymer (91/5.5/3.5) Example 2
Ethylene-butyl EVA modified EVOH modified EVA acrylate- EVA EVA
maleic anhydride co- polymer (91.5/5.3/3.0)
__________________________________________________________________________
Thereafter the melt is quenched to form a tape. This tape is
electronically cross-linked and successively oriented to obtain a
tubing. This tubing is internally Corona treated and converted into
bags.
In the following Table 2 the physical properties of the films
according to the comparative example and Examples 1 and 2 according
to the present invention are summarized.
TABLE 2 ______________________________________ Comparative
Properties Example Example 1 Example 2
______________________________________ Partial Thickness (.mu.m) A
14 16 15 B 21 13 13 C 4 4 4 D 5 3 3 C 5 4 4 B 21 22 22 Total
Thickness (.mu.m) 67 61 63 Modulus kg/cm L 22.9 10.0 11.3 at
23.degree. C. (actual) T 26.3 10.9 10.4 kg/cm.sup.2 L 3627 1816
1757 T 3756 1624 1724 Tensile kg/cm L 3.2* 2.6 2.9 Strength
(actual) T 3.8 2.6 (const.) 2.9 kg/cm.sup.2 L 493* 470 493 T 529
437 516 Elongation % L 156* 158 166 T 115 140 119 L T L T L T
Shrink % at 75.degree. C. 9 20 14 19 15 20 80.degree. C. 13 25 21
28 21 26 85.degree. C. 19 32 27 36 28 37 90.degree. C. 28 40 42 51
40 47 95.degree. C. 41 51 53 59 51 58 Shrink kg/cm 0.08 0.16 0.09
0.11 0.08 0.11 Tension kg/cm.sup.2 11.4 22.0 14.7 19.4 14.2 19.4
Haze % 9.5 5.6 7.7 Gloss (i = 60 degrees) 98 96 85 gloss units
______________________________________ Remarks:? (*)Breakage of
layers DC-B occurs before the total breakage
From the above Table 2 it can be taken that the shrink films
according to the examples of the present invention provide a better
free shrink performance than the shrink film according to the
comparative example.
From the bags manufactured from the multilayer shrink films
according to the Comparative Example and Examples 1 and 2 of the
invention, bags of a size of 300.times.500 mm have been
manufactured. The bags have been sealed and the seal strength has
been measured by means of the "Parallel Plate Test" both for the
untreated film and the film treated by means of a Corona treatment.
In the "Parallel Plate Test" each bag was clamped in a fixture
provided with a hose. The open mouth end of the bag was clamped
around the hose. Air was pumped through the hose whereby the bag
was inflated. The two sides of each bag were respectively
restrained by two metal plates based about 10 cm apart. For each
bag the pressure was increased via the hose at the rate of 1 inch
of water pressure per second until the seal for that bag burst open
at the inches of water pressure designated in Table 3 below.
Furthermore, the meat adhesion results of the bags obtained have
been tested for the untreated and the Corona treated films.
Finally, a Cooking Test has been carried out showing the
temperature at which seal reopening occurs. The data obtained in
these tests are summarized in the following Table 3, wherein the
data with respect to the seal strength and the meat adhesion are
shown relatively to the results obtained with the material of the
Comparative Example, which--as far as the Relative Meat Adhesion
Test is concerned--has been normalized to be 100.
The following test method has been developed to measure the degree
of meat adhesion of various experimental or other products in
comparison with an internal standard material (the comparative
example of following Table 3).
A large piece of hairless pork skin ("pork rind") is washed with
detergent, rinsed and dried. A series of strips of rind of 18
cm.times.15 cm is cut, aiming to be as uniform as possible. Each
strip of pork rind is then wrapped around a piece of corrugated
cardboard. These pieces of pork rind are then placed in contact
with a piece of standard material on one side and an experimental
material on the other side. The pork rind and adhering plastic film
are then clamped between two thick aluminum plates and placed in a
thermostatic cooking bath for 1.5 hours at 70.degree. C.
At the end of the cooking cycle the whole assembly is cooled in
cold water, and then the samples are removed from the cell.
The individual strips of packaging material are then peeled off the
pork rind and the force required to do so is measured by
dynamometer in each case, for each strip of rind, a comparative
value for the peel force is measured in comparison with the
standard material.
TABLE 3
__________________________________________________________________________
Cooking Test Temperature Seal Strength at which seal (inches of
water Relative Meat reopening Property pressure) Adhesion occurs
(.degree.C.)
__________________________________________________________________________
Comparative untreated 130 100 70 Example Example 1 untreated 170
330 75 Example 2 untreated 172 200 70 Example 1 Corona treated 175
300 95 Example 2 Corona treated 186 230 90
__________________________________________________________________________
From the above Table 3 it can be seen that the bags obtained from
the multi-layer shrink film according to the present invention
provide a good thermal resistance, as measured by a cooking test,
in which the temperature at which the seal is open is measured.
Furthermore, the internal corona treated version of the invention
film unexpectedly provides a significant improvement in the thermal
resistance. The products of examples 1 and 2 in the corona treated
version resist for 20.degree. to 25.degree. C. more than the
non-treated product. This is contrary to previous experience well
known to those skilled in the art, that sealability and/Or sea1
strength of polymer materials such as polyethylene or polypropylene
is normally diminished by corona treatment.
It will also be noted from Table 3 that meat adhesion of these
materials is not significantly affected by corona treatment, which
may be an advantage in that too high meat adhesion may cause damage
to the meat structure when the packaging material is removed from
the meat product.
Although the present invention has been described in conjunction
with preferred embodiments, it is to be understood that
modifications and variations may be utilized without departing from
the principles and scope of the invention, as those skilled in the
art will readily understand.
* * * * *